Stirling piston rings

[spinningmagnets]

I am curious about the current "go to" piston ring for a large working Gamma. The small brass desktop demonstrators seem to have no seal, or use a rubber O-ring.

The 1880's Rider-Alpha used leather seals that were lightly oiled, and the design simply accepted a certain amount of air leakage, so it incorporated an air-pump and relief valve that kept the internal gas at 2-atmospheres. The later versions water-cooled the seal area on the hot end to help the seal last longer.

I was scrolling steam-engine forums and found a long post on piston rings. The builder recommended 25%-35% graphite impregnated PTFE "Teflon" rings with a stepped cut (step-cut ring-gap shown in pic below). I don't know if these can be purchased or if they have to be machined from raw stock. I know steam is different than hot/dry air, but I can't help but to be curious. Bronze was listed as a desirable material for steam pistons and cylinders, due to rust on steel. For a dry-air Stirling this wouldn't be an issue, although a bronze piston in a steel cylinder does sound intriguing.

That type of graphite/PTFE ring-seal was listed as good up to a temperature of 250C (480F). I would appreciate any information on seal types that have been tried, and the results.

A

RingPistonStepCut.png (134.32 KiB) Viewed 3118 times

[Fool]

If you are going with a conventional piston and rings, do a search on gapless rings. Here is a commercial link:

https://www.totalseal.com/?utm_source=d ... gLnrfD_BwE

It may be inspirational?

I was wondering if a graphite piston could incorporate graphite piston rings? They are very brittle. Graphite is self lubricating on some materials and high heat to boot.

Also the following thread has some excellent points about rings, if you haven't already read it:

viewtopic.php?p=18878&hilit=Translate#p18878

[staska]

I am curious about the current "go to" piston ring for a large working Gamma. The small brass desktop demonstrators seem to have no seal, or use a rubber O-ring.

The 1880's Rider-Alpha used leather seals that were lightly oiled, and the design simply accepted a certain amount of air leakage, so it incorporated an air-pump and relief valve that kept the internal gas at 2-atmospheres. The later versions water-cooled the seal area on the hot end to help the seal last longer.

I was scrolling steam-engine forums and found a long post on piston rings. The builder recommended 25%-35% graphite impregnated PTFE "Teflon" rings with a stepped cut (step-cut ring-gap shown in pic below). I don't know if these can be purchased or if they have to be machined from raw stock. I know steam is different than hot/dry air, but I can't help but to be curious. Bronze was listed as a desirable material for steam pistons and cylinders, due to rust on steel. For a dry-air Stirling this wouldn't be an issue, although a bronze piston in a steel cylinder does sound intriguing.

That type of graphite/PTFE ring-seal was listed as good up to a temperature of 250C (480F). I would appreciate any information on seal types that have been tried, and the results.


RingPistonStepCut.png

Or just buy ready made 63.5 / 69 mm diameter ptfe cup ring and mating cylinder from china. Used in 99% of cheap air compressors. After some heating or work-in - close to zero friction.

[spinningmagnets]

Thank you, staska!

[jpigg55]

Something I'm wanting to try is High-Temperature Graphite-Filled 841 Bearing Bronze (https://www.mcmaster.com/products/metal ... onze-rods/) machined into Clupet piston rings (https://www.youtube.com/watch?v=80jzsNM6olE).

[spinningmagnets]

From VincentG...

"...For what it's worth, the Essex mostly solved the hot piston sealing issue. The piston dome was thin brass and only .015" or so smaller than the bore, but protrudes up a few inches, preventing the majority of the hot gas from reaching the piston sealing area...."

The Essex piston is somewhat long, and it sounds like the piston ring is around its base, rather than the tip. Any hot air that squeezes by the snug fit of the piston would be small enough for the finned cylinder to shed the majority of the heat before reaching the seal.

This also reminds me of the Rider-Alpha. The pistons are also long, and even though the seals are attached to the cylinder instead of the piston, they are located as far as possible from the hottest part of the piston. Also, on the more refined later versions of the Rider-Alpha, the hot-end seal was externally water-cooled to aid longevity. Of course it was leather instead of teflon...

[spinningmagnets]

jpigg55, I've never heard of Clupet rings before. Now that I've seen them, I don't know why they aren't more common! Thanks.

A

PistonRingClupet.png (368.77 KiB) Viewed 3045 times

[VincentG]

The Essex piston is somewhat long, and it sounds like the piston ring is around its base, rather than the tip. Any hot air that squeezes by the snug fit of the piston would be small enough for the finned cylinder to shed the majority of the heat before reaching the seal.

This also reminds me of the Rider-Alpha. The pistons are also long, and even though the seals are attached to the cylinder instead of the piston, they are located as far as possible from the hottest part of the piston. Also, on the more refined later versions of the Rider-Alpha, the hot-end seal was externally water-cooled to aid longevity. Of course it was leather instead of teflon...

Good point, very similar to Rider Alpha.

While they suck for ICE engines, modern automotive "low tension" rings might adapt well to Stirling engines, being specifically designed for low friction.

[staska]

Btw - for 69mm cylinder one can buy 68mm diametr cup holder from stainless stell. It fits with good gap for hot cap. Sad, its only limited for 30mm stroke or a little more. Cylinders are 47.5mm long, skirts is 5mm height.

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Screenshot_2024-06-19-19-16-38-615_com.alibaba.aliexpresshd-edit.jpg (274.67 KiB) Viewed 3029 times

[jpigg55]

jpigg55, I've never heard of Clupet rings before. Now that I've seen them, I don't know why they aren't more common! Thanks.

Glad you found them of interest. I stumbled on them when considering making a steam engine, but couldn't find any satisfactory info on making a boiler for it.
I think the reason for not being more common is twofold.
More labor intensive since I don't think they can be made via CNC and, as far as I can tell, limit to old steam engine use.
I just thought they would be a good fit for Alpha type Stirling engines.

[spinningmagnets]

From "Making Stirling Engines" by Andy Ross (3rd ed. 1997)

pg 11, the 65cc rhombic was rebuilt...Its friction was low and its seals were excellent. Both the piston and the displacer used two-cycle engine racing·type piston rings, which are thin chrome plated steel rings with very low outspring.

pg 13, a 300cc test engine [was] built under a DOE appropriate technology grant to make a simple, low pressure, high speed hot air engine of 100 watts output...The DOE engine used off-the-shelf piston rings (re-machined for lower outspring and friction)...It produced 112.4 watts peak power at 1150 rpm

[spinningmagnets]

I forgot to add from the Andy Ross book...he did use a long power-piston with the seals/rings at the base instead of the crown (as mentioned before in this discussion), and one interesting bit is that the piston itself can absorb heat which travels down the sides of the piston, which adds heat to the area where the rings live.

He added a disc of insulation on the crown of the piston, then put a thin metal cap on it to protect the insulation. This won't stop the piston from eventually getting very warm if put under load for an extended amount of time, but it can slow the heat uptake into the piston so that "some" of the heat can be shed along the way, which leaves the piston ring area becoming warm instead of hot.

High-performance gasoline engines sometimes have a jet of oil that is sprayed onto the underside of the piston on the exhaust side. I wonder what I can do to help the piston shed heat? Perhaps a jet of air? Probably more trouble than its worth...I'm thinking the long piston with insulated cap would be the most useful effort, along with a cylinder that has cooling fins around its base (like the Essex).